Polyurethanes are used for a wide range of commercial applications such as adhesives, coatings on different substrates including textiles, plastics, wood, glass fibers, metals and the like due to their extremely good chemical and abrasion resistance, touglness, flexibility and durability. Conventionally, these polyurethane coating products are diluted with organic solvents to get desirable consistency before application. As the coating dries, these organic solvent vapors escape into the atmosphere. This is both economically and environmentally disadvantageous on account of the higher cost of organic solvents, and the pollution and health hazards caused by such fugitive emissions. More importantly, these organic vapors pollute the atmosphere and create serious health hazards.
Many approaches are reported in prior art to form polyurethane coatings from the dispersion of these polymers in water. The use of water is economically advantageous as well as it does not pollute the atmosphere. However, polyurethane polymers are not compatible with water i.e they do not form stable dispersions in water until special process and/or special monomer(s) are used in their preparation.
One such approach in the prior art is the use of external emulsifiers to disperse and stabilise the polymers in water (U.S. Pat. No. 2,968,575). Though these emulsifiers stabilise the polymer in water, they posses the disadvantage that such coatings are themselves sensitive to water, and hence give poor coating performance in terms of hydrolytic stability. To overcome the above draw back, ionic groups are incorporated into the polyurethane backbone during polymerisation Upon neuralising these ionic groups the respective salts formed act as an internal emulsifier. One such approach (Neth. Pat. Appln. 6,410,928) discloses the use of cationic ionomers for preparing polyurethane latexes. In another approach (Ger. Offen. 2,744,544) polyurethane production was disclosed using anionic ionomers. The incorporation of nonionic emulsifiers in the polyurethane back bone is disclosed in U.S. Pat. No. 4,190,566. Further improvements in properties of coating derived from polyurethane dispersion are obtained by combining both ionic and non-ionic group in the polymer back bone. This has been disclosed in U.S. Pat. No. 4,238,378.
These prior art patents are related to the production of linear polyurethane dispersions. Coatings derived from these linear polyurethane dispersions possess limited water resistance and do not have good solvent resistance.
Ger Pat. 4,237,965 discloses the production of aqueous polyurethane binders from dimer diol and trimethylol propane as isocyanate reactive group for stoving lacquers. Herein, branching in the diol is expected to increase water resistance along with other important properties. Nevertheless, the externally added short chain triol increases the prepolymer viscosity, resulting in the reduction of the solid content of the dispersion to about 25%. Jap.Pat. 06,329,744 and Jap.Pat. 06,93,068 disclose the production of water dispersible polyurethane resin using trimethylol propane along with other isocyanate reactive components. The externally added short chain triol contributes more to the hard segment of the polymer. Further, crosslinking with aminoplast resins results in coatings with diminished flexibility and more prone to cracking.
Polyurethane dispersions disclosed in the prior art are normally derived from linear diols as one of the isocyanate reactive component and some quantity of short chain triols to impart a certain degree of branching/crosslinking . The main disadvantage of using short chain triols externally are a) undesirable increase in viscosity during prepolymer production b) insufficient branching/crosslinking c) short chain triols contribute more to hard segment and cracking of film is observed d) poor hydrolytic stability.